I'll go a little more in depth as to why this is:
Lithium Ion batteries are different form older batteries in that they don't really need the full discharge to full charge that most people became accustom to. (Granted people say its a good idea for the first few charges, but I think it's mostly superstition at this point)
So Apple says that the iPhone battery has something like 300 charge cycles (or whatever it really is). In the old mentality that means 100% to 0% to 100% would be 1 charge cycle. however most people don't let their phones do that, so lithium ion batteries were also designed to count it as follows:
100% to 50% to 100% to 50% to 100%. If you follow that, using it to half charge, then back to full charge, then again using to half charge, and finally a full charge. This would ALSO count as 1 charge cycle.
So here's why I say to keep something plugged in as often as possible. Once at full charge, if you keep most electronics plugged in, the drain on the battery is much less while plugged in than if you were to only plug in to charge. The power transfer is designed like this to minimize battery drain.
Since I don't really know the iPhone design very well I can only speculate but I would assume well over 100+ hours of use while plugged in would equal 1 charge cycle on the battery.
Thats the basic rundown. If anybody else has some insight I would love to hear it.